Relatively large phased array antenna architectures, such as but not limited to spaceborne, chirped synthetic aperture radar systems, typically contain a multiplicity of transmitters and receivers distributed across respective spaced apart arrays. In such systems, a common, very precise reference frequency signal is customarily supplied to both the transmit and receive array portions. As such, there is the issue of how to take into account phase shift associated with variations in the substantial length of signal transport cable that links the reference frequency source, which is customarily installed in one location of the array, with the remaining portion of the array.
Because terrestrial open loop calibration of the system suffers from the inability to take into account variation in temperature along the transport cable due to changes in sun angle, and variations in obscuration by components of the antenna support platform in the antenna's space-deployed condition, it has been proposed to perform temperature measurements at a number of locations along the cable and provide phase compensation based upon the measured values. A drawback of this approach stems from the fact that there are non-linearities within the cable, so that over different temperatures it is necessary to employ a larger number of values in the calibration table. In addition, because this technique performs multiple measurement points along the cable, it introduces associated variations in loading which, in turn, produce separate amounts of phase shift to the reference frequency signal.
In accordance with the invention disclosed in the above-referenced '843 application, this transport cable-based phase variation problem is effectively obviated by injecting an RF chirp signal into the signal cable from the remote end thereof, and correlating the returned chirp that is reflected from the reference source end with a delayed version of the injected chirp, to derive a measure of the phase delay through the cable between its opposite ends.
Although this approach works quite well for a single length of cable, it can become cumbersome when applied to a multinode system, wherein the reference signal is to be delivered to a plurality of spatially separated array sites. One straightforward approach would be to implement a star-configured architecture, with each spoke of the star containing its own dedicated chirp generator and associated processing circuitry. Unfortunately, such an approach is hardware intensive, and costly to implement.